In a typical papermaking process, water is extracted from the paper web or mat in a number of different operations. For example, in the forming section the paper web is conveyed on a forming wire or belt across a series of spaced transversely extending foil blades. Each foil blade is provided with a generally sharp edge facing in an upstream direction and as the paper web passes across the foil blade, a partial vacuum is created between the web and the upper surface of the blade, which draws water from the web. The sharpened edge of the next succeeding blade riding against the lower surface of the wire will scrape and remove the moisture from the wire.
Water is also extracted from the paper web through use of suction or Uhle boxes. The typical suction or Uhle box includes a rigid cover which supports the wire that carries the paper web, and the cover is provided with a plurality of perforations or slots. By drawing a vacuum on the interior of the box, water is drawn from the paper web into the box.
Water extraction can also be achieved by passing the paper web over a perforated roll, while subjecting the web to a pressure differential created by exposing the interior of the roll to a vacuum.
In other papermaking applications, water is extracted from the web by passing the paper web, while sandwiched between a pair of porous belts or wires, around a curved shoe. As the paper web confined between the twin wires passes around the curved surface, a compressive force is exerted along with centrifugal force which tends to drive the water outwardly from the web.
In a dryer section of a typical papermaking machine, the paper web is passed over a series of heated dryer rolls to drive off the moisture, and in some applications, the dryer rolls may be perforated and a pressure differential applied to pass air through the web to facilitate the drying operation.
The water extracting media, such as suction and Uhle box covers, perforated drums, and the like are generally composed of rigid plastic, metal, or solid ceramic and slots, holes or grooves are machined in the material. With this type of construction, the holes or slots are relatively large in size and when the material in use is subjected to a pressure differential, the fiber material of the paper web can be drawn into the holes or slots, causing fiber misorientation in the web. A further disadvantage is that the relatively large sized openings, i.e. holes and/or slots, tend to shadow mark the web, which would make the paper unsuitable for certain applications. Moreover, in the conventional water extraction operation, the pressure differential is applied only through the openings and consequently, the water drainage at any particular instant is through isolated areas of the web, which results in non-uniform water extraction.